Helium runaways in white dwarfs

The long term evolution of an accreting carbon white dwarf was studied from the onset of accretion to the ignition of helium. The variations in the details of the helium shell flash examined with respect to variations in mass accretion rate. For intermediate rates the helium flash is potentially explosive whereas for high rates the shell flash is relatively weak. The results are discussed in the context of the long term evolution of novae

The MXB1916-053/4U1915-05: Burst properties and constraints on a 50 minute binary secondary

Results are presented from OSO-8 and HEAO-1 A2 observations of 34 bursts from the X-ray burster MXB1916-053/4U1915-05 recently discovered to show a 50 minute binary period. While 11 burst previously reported all had similar light curves, 22 observed two years later show a factor of 3 range of peak fluxes and decay times between 3 and 20 s. Recurrence times between successive bursts vary between 3 and 6 hours. A ratio of steady flux to average burst flux of equiv 120 is developed. A burst observed with the HEAO-1 A2 experiment showed an initial temperature rise to a peak black body temperature of equiv 3 keV followed by the cooling typical of type I bursts. The burst was unusual in that the apparent projected size of a blackbody source increased by a factor of 3 during the cooling phase

The Impact of Type Ia Supernova Ejecta on Binary Companions

We present adaptive mesh refinement (AMR) hydrodynamical simulations of the interaction between Type Ia supernovae and their companion stars within the context of the single-degenerate model. Results for 3D red-giant companions without binary evolution agree with previous 2D results by Marietta et al. We also consider evolved helium-star companions in 2D. For a range of helium-star masses and initial binary separations, we examine the mass unbound by the interaction and the kick velocity delivered to the companion star. We find that unbound mass versus separation obeys a power law with index between -3.1 and -4.0, consistent with previous results for hydrogen-rich companions. Kick velocity also obeys a power-law relationship with binary separation, but the slope differs from those found for hydrogen-rich companions. Assuming accretion via Roche-lobe overflow, we find that the unbound helium mass is consistent with observational limits. Ablation (shock heating) appears to be more important in removing gas from helium-star companions than from hydrogen-rich ones, though stripping (momentum transfer) dominates in both cases.Comment: 6 pages, 2 figures, to appear in the proceedings of the conference "Binary Star Evolution: Mass Loss, Accretion, and Mergers" at Mykonos, Greece, June 22-25, 201

Constraints on Thermal X-ray Radiation from SAX J1808.4-3658 and Implications for Neutron Star Neutrino Emission

Thermal X-ray radiation from neutron star soft X-ray transients in quiescence provides the strongest constraints on the cooling rates of neutron stars, and thus on the interior composition and properties of matter in the cores of neutron stars. We analyze new (2006) and archival (2001) XMM-Newton observations of the accreting millisecond pulsar SAX J1808.4-3658 in quiescence, which provide the most stringent constraints to date. The X-ray spectrum of SAX J1808.4-3658 in the 2006 observation is consistent with a power-law of photon index 1.83\pm0.17, without requiring the presence of a blackbody-like component from a neutron star atmosphere. Our 2006 observation shows a slightly lower 0.5-10 keV X-ray luminosity, at a level of 68^{+15}_{-13}% that inferred from the 2001 observation. Simultaneous fitting of all available XMM data allows a constraint on the quiescent neutron star (0.01-10 keV) luminosity of L_{NS}<1.1*10^{31} erg/s. This limit excludes some current models of neutrino emission mediated by pion condensates, and provides further evidence for additional cooling processes, such as neutrino emission via direct Urca processes involving nucleons and/or hyperons, in the cores of massive neutron stars.Comment: 5 pages, 2 figures; slight revisions, accepted by Ap